Apparatus for applying molten wax onto printed circuit board

ABSTRACT

An apparatus for applying molten wax onto a printed circuit board travelling along a predetermined transfer path. The applied wax after solidification serves to fix the electric components mounted on the board, facilitating the succeeding operations such as cutting and soldering. The apparatus includes a vessel for containing a molten wax, a plurality of ejection nozzles arranged in a direction transverse to the transfer path, valve means connected to each of the ejection nozzles and operable so that the molten wax is ejection from each nozzle upon actuation of the corresponding valve means, and a pump, such as a gear pump, having a suction side in fluid communication with the vessel and a discharge side in fluid communication with each of the valve means. Means are provided for selectively rendering the valve means operable according to the width of the printed circuit board.

BACKGROUND OF THE INVENTION

This invention relates generally to the processing of printed circuitboards having mounted thereon electric components for forming goodsolder joints. More particularly, the present invention is concernedwith an apparatus for applying molten wax over the component side of theprinted circuit board, the applied wax after solidification serving as abond for the fixation of the electric components to the printed circuitboard.

For the purpose of providing uniform, steady joints between componentleads and circuits printed on the board, a technique is proposed inwhich the electric components-bearing board is first subjected to a dipsoldering treatment for providieng temporary joints between the electriccomponents and the board and the resultant board, after cutting theleads depending from the underside of the board, is contacted with astanding wave of molten solder ejected from a nozzle.

This technique, however, suffers from drawbacks that the quality of theelectrical components may be deteriorated through the double solderingtreatments and the apparatus becomes large in structure. Moreover, thecontact of the temporarily joined components with the ejected, runningmolten solder may cause the leads to come off from the through holes ofthe board.

SUMMARY OF THE INVENTION

The present invention provides an apparatus for applying molten wax ontoa printed circuit board travelling along a predetermined transfer path,which comprises a vessle for containing a molten wax, a plurality ofejection nozzles arranged in a direction transverse to the transferpath, valve means connected to each of the ejection nozzles and operableso that the molten wax is ejected from each nozzle onto the printedcircuit board travelling adjacent thereto upon actuation of thecorresponding valve means, a pump having a suction side in fluidcommunication with the vessel and a discharge side in fluidcommunication with each of the valve means, and means for selectivelyrendering the valve means operable according to the width of the printedcircuit board.

It is, therefore, an object of the present invention to provide anapparatus for applying molten wax onto printed circuit boards travellingalong a predetermined transfer path.

Another object of the present invention is to provide an apparatus ofthe above-mentioned type which is simple and compact in construction andwhich can apply a controlled amount of the molten wax onto the printedcircuit boards.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome apparent from the detailed description of the preferredembodiments of the invention which follows, when considered in light ofthe accompanying drawings, in which:

FIG. 1 is a flow diagram schematically showing a series of work stationsarranged for soldering printed circuit boards, including a wax-applyingstation according to the present invention;

FIG. 2 is a perspective view, partly cut away, diagrammatically showingone embodiment of the wax-applying apparatus according to the presentinvention;

FIG. 3 is an exploded, explanatory view showing a gear pump of FIG. 2;

FIG. 4 is a fragmentary, cross-sectional view taken on line IV--IV ofFIG. 2;

FIG. 5 is a vertical, cross-sectional view diagrammatically showing oneembodiment of an ejection nozzle member according to the presentinvention; and

FIG. 6 is a front view, partly in cross-section, diagrammaticallyillustrating the manner of applying molten wax onto the printed circuitboard by means of the invention apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Referring first to the flow diagram of FIG. 1, a printed circuit board10, having the leads 12 of electric components 11 inserted therethroughis conveyed along a predetermined path 18 by means known per se such asa sprocket and chain mechanism. During its passage along the path 18,the board 10 is subjected to a series of treatments for firmly unitingthe circuit printed on the board 10 with the leads 12 of respectivecomponents 11.

The board 10 passes first through a wax-applying apparatus 24 whereinthe component side of the board 10 is applied with a molten wax injectedfrom a plurality of transversely arranged injection means 36. Any wax,such as vegetable wax and petroleum wax, having a softening pointgenerally 60° to 100° C. may be suitably used for applying onto theprinted circuit board. The molten solder applied onto the board 10 andcovering the components 11 solidifies during the passage of the board 10through a cooler 13 so that the components 11 are fixed on the boardwith the space around the components 11 and the through holes of theboard for receiving the leads 12 being filled with the solidified wax.

The leads 12 depending from the underside of the board 10 are cut by acutter 14. Thereafter the underside of the board 10 is evenly appliedwith a liquid flux during the passage thereof above a fluxer 15 ispreheated generally to 80°-90° C. by a preheater 16 to condition samesuitable for receiving molten solder in the succeeding soldering station17.

An embodiment of the wax-applying apparatus 24 is shown in FIG. 2. Thewax-applying appartus 24 includes a vessel 26 for containing a moltenwax 25 maintained at a suitable temperature by means of a temperaturecontrol unit 38. A pump such as a gear pump 27 is received in the bottomof the vessel 26. The pump 27 has a suction side in fluid communicationwith the wax vessel 26 and a discharge side in fluid communication witha U-shaped fluid flow passage 30 defined in the body of a manifold 32. Adrive shaft 41 securing a drive gear (not shown) extends from the pumphousing and is connected to a drive motor 28.

A plurality of pipes 33 are connected at their one ends to respectiveports of the manifold 32 for fluid communication with the U-shapedpassage 30. The other end of each of the pipes 33 is connected to anejection means 36. Thus, the gear pump 27 is operated by the actuationof the motor 28 to feed the molten wax 25 in the vessel 26 to theejection means 36 through the fluid flow passage 30 of the manifold 32and pipes 33 as shown by the arrow in FIG. 2, the molten wax beingejected from the ejection means 36 toward the printed circuit board (notshown) travelling adjacent thereto.

Indicated as 31 is a filter or strainer to prevent foreign solids fromflowing therethrough. The filter 31 may be removed from the manifold 32through a capped port 39 for cleaning. Each pipe 33 is surrounded by aheater element 34 such as a tape heater to maintain the wax flowingthrough the pipe 33 in the molten state. The heater element is coveredwith a suitable heat insulating sheath 35.

FIG. 3 depicts the construction of the gear pump 27 by way of anexploded view. A pair of externally meshed gears 46 and 47 areaccommodated in a gear case 48. The gear case 48 has a pair of axialgrooves 48a and 48b formed at its inside wall so that there are defined,when the case 48 receives the gears 46 and 47, suction side anddischarge side chambers between the grooves 48a and 48b and the gears 46and 47. The gear case is sandwiched between an upper plate 45 providedwith an inlet opening 45a and a lower plate 49 provided with an outletopening 49a and a return opening 49b.

The lower plate 49 has a pair of bores 49c and 49d for rotatablyreceiving the gear shafts 41 and 42 which are inserted through centralthrough holes of the gears 46 and 47 and fixedly secured to the gears 46and 47 for rotation therewith, respectively. The upper plate 45 isformed with a pair of through holes 45c and 45d for the insertion of theshafts 41 and 42 therethrough, respectively.

The upper and lower plates 45 and 49 and the gear case 48 are eachprovided with a pair of opposite guide holes 45e, 49e and 48e, and alsowith four angularly equally spaced apart through holes 45f, 49f and 48f,respectively, for the insertion of four fixing screws 50 therethrough.Preferably, the upper plate 45 is overlaid with a cover member 44provided with an opening or a concaved portion 44a for opencommunication with the inlet opening 45a of the upper plate 45. Thecover 45d is formed with a pair of guide openings or grooves 44c and 45dthrough which are inserted the shafts 41 and 42 to prevent the lateralmovement of the gear shafts and to ensure smooth rotation of the gearshafts. The cover 44 is also provided with opening 44f for the insertionof the fixing screws 50.

As will be seen from FIG. 3, the through holes of the upper and lowerplates 45 and 49, gear case 48 and the cover 44 are positioned so thatthe two gear shafts 41 and 42, two guide bars 43 and four fixing screws50 can be simultaneously inserted in respective through holes to form asuction side fluid passage defined by the opening 44a, inlet opening 45aand the groove 48a and a discharge side fluid passage defined by thegroove 48b and outlet opening 49a, with the return opening 49b being influid communication with the suction side flow passage.

In constructing the gear pump 27, the lower plate 49 is first placed onthe manifold 32 so that the outlet opening 49a faces on the open end ofthe fluid flow passage 30 of the manifold 32. The gear shafts 41 and 42and the two guide bars 43 are then inserted into the bores 49c, 49d and49e, respectively. The gears which are meshing engagement with eachother are fixedly mounted on the shafts 41 and 42, respectively, by anysuitable means such as set screws. The gear case 48, upper plate 45 andcover 44 are then mounted on the lower plate 49. The entire structure issubsequently unitedly fixed by means of the screws 50 to the manifold32. The motor 28 is finally mounted on the vessel 26 with its driveshaft being connected to the gear shaft 41 (see FIG. 2).

In order to control the flow rate of the molten wax through respectivepipes 33, the wax-applying apparatus 24 of the present invention isdesirably provided with a flow control valve. FIG. 4 shows a preferredembodiment of the flow control valve 29 fitted in a bore of the manifold32. The valve 29 has a valve casing 61 meshed into a bore of themanifold 32. The valve casing 61 is provided with an inlet port 67 at aposition to communicate with the fluid flow passage 30 of the manifold32 and an outlet port 69 at a position to communicate with a returnpassage 52 which is formed within the manifold 32 and which is in fluidcommunication with the return opening 49b.

A valve stem 62 which is threaded into the casing 61 is movable in axialdirection by rotation thereof together with its valve plug 62a so thatthe gap between a valve face 62b of the valve plug 62a and a valve seat64 of the valve casing 61 can be adjusted to control the flow rate ofthe molten wax flowing through the return passage 52, i.e. the flow rateat respective ejection means 36.

The flow control valve 29 of FIG. 4 is further provided with a safetyvalve 63 which serve to maintain the pressure within the fluid flowpassage 30 below a predetermined value. The vavle casing 61 has an openend threadedly provided with an inlet mouth 68 which opens into one endof the U-shaped fluid flow passage 30 and is formed with an annularvalve seat 68a. A safety valve plug 65 is provided for axial movementwithin the casing 61 and has an end face 65a abutting engagement withthe valve seat 68a. The valve plug 65 is constantly pressed by a coilspring 66 to urge the valve face 65a in engagement with the valve seat68a. The biasing force of the coil spring 66 may be adjusted by turningthe mouth 68 in a suitable direciton. Indicated as 70a, 70b, 70c and 70dare sealing means such as O-rings.

The flow control valve 29 with a built-in safety valve 63 is operated asfollows. Upon rotation of the drive shaft 41 by actuation of the motor28, the gears 46 and 47 are rotated so that the molten wax in the vessel26 passes successively through the opening 44a of the cover plate 44,the inlet opening 45a of the upper plate 45, the groove 48a of the gearcase 48, the groove 48b of the gear case 48, the outlet opening 49a ofthe lower plate 49 and the inlet port 51 communicated with the U-shapedfluid flow passage 30 of the manifold, as shown by the arrows A and B.The molten wax introduced into the passage 30 is then flown through thepipes 33 ejected from the ejection means 36 upon actuation thereof. Inthis case, a portion of the molten wax within the fluid flow passage 30is recycled to the suction side of the pump 27 through the gap betweenthe valve seat 64 and the valve face 62b of the control valve 29, thedischarge port 69, the return passage 52 and the return opening 49b asshown by the arrows E and G in FIG. 4, the amount of the recycled waxbeing adjustable by the control of the valve 29. Therefore, the flowrate of the molten wax ejected from the ejection means 36 may becontrolled by the valve 29. Since the amount of the molten wax appliedonto the printed circuit board is desired to be changed according to thekind of electric components mounted thereon and since the number of theejection means rendered operable is desired to be varied according tothe width of the printed circuit board, the provision of the flowcontrol valve 29 is very preferable.

The pressure within the passage 30 increases when the ejection meansstop operating. When the pressure exceeds a predetermined value, theplug 65 is displaced against the biasing force of the spring 66 todirect a portion of the wax in the passage 30 through the safety valveto the return passage 52 as shown by the arrows F and G. By turning themouth 68, the pressure at which the safety valve actuates may beadjusted.

FIG. 5 illustrates an example of the ejection means 36. The ejectionmeans 36 includes a valve casing 71 to which are connected the pipe 33and a nozzle member 78. Indicated as 74 is a valve stem having its oneend formed with a valve face 75 and the other end securing a piston 85slidably held in air cylinder 84 supported by a stationary support (notshown). The valve face 75 is engageable with a valve seat 76 formedwithin the valve casing 71 to prevent fluid flow from the pipe 33 to thenozzle 78.

The valve stem 74 is fitted with a coil spring 82 so that the valve face75 normally engages the valve seat 76. When the air cylinder 84 isactuated to dispace its piston 85 upward, the valve stem 74 connected tothe piston 85 moves therewith, whereby the valve face 75 is disengagedfrom the valve seat 76. Thus, during the operation of the air cylinder84, the molten wax is ejected from the nozzle member 78 towards theprinted circuit board (not shown).

It is preferred that a plurality of the ejection means be provided andarranged so that the nozzle members may be positioned in a directiontransverse to the transfer path of the printed circuit board. In such acase, it is also preferred that selective number of thus arrangedejection means be actuated correspondingly to the width of the printedcircuit board. FIG. 6 shows an embodiment of such an arrangement, inwhich the similar components are designated by the same referencenumerals. The reference numerals 21a and 21b designate a pair ofstationary and moveable base members each supporting thereon a sprocketand chain mechanism for conveying printed circuit boards 10.

The printed circuit boards 10 are supported at their both side ends by aplural pairs of holding members 91a and 91b respectively attached to apair of parallel, endless transfer chains 22a and 22b. Such a mechanismfor transferring the printing circuit boards is well known in the art.

The base members 21a and 21b are interconnected by an adjusting screw92. One end of the screw 92 is rotatably received by the stationary basemember 21a while the other end is in threaded engagement with themoveable base member 21b. By rotation of the screw 92, the moveable basemember 21b is displaced toward or away from the stationary base member21a according to the direction of rotation of the screw 92 so that thedistance between the holding members 91a and 91b can be adjusted to avalue suitable for supporting the printed circuit board therebetween.Indicated as 23a and 23b are guides provided on respective base members21a and 21b for maintaining 22a and 22b in proper parallel orientation.

The moveable support member 21b is provided with an actuation member 93arranged for engaging with a plurality of limit switches 94a₁, 94b₂, . .. 94a_(n) electrically coupled with respectrive ejection means 36a₁,36a₂, . . . 36a_(n). The ejection means 36a₁, 36a₂, . . . 36a_(n) arerendered operable only when their corresponding limit switches are in ONposition by contact with the actuation member 93. Thus, each time theprinted circuit board is positioned at a predetermined location, theselected number of the ejection means (four in this illustrated case)are simultaneously operated to eject molten wax 25 towards the printedcircuit board 10, the number being automatically determined according tothe position of the moveable base member 21b, i.e. the distance betweenthe opposite holding members 91a and 91b, i.e. the width of the printedcircuit board 10 to be processed.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all the changes which come within the meaning and rangeof equivalency of the claims are therefore intended to be embracedtherein.

I claim:
 1. An apparatus for applying molten wax onto a printed circuitboard travelling along a predetermined transfer path, comprising avessel for containing a molten wax, a plurality of ejection nozzlesarranged in a direction transverse to the transfer path, individualvalve means connected to each of the ejection nozzles and operable sothat the molten wax is ejected from each nozzle onto the printed circuitboard travelling adjacent thereto upon actuation of the correspondingvalve means, a pump having an intake in fluid communication with thevessel and a discharge in fluid communication with each of the valvemeans, means for selectively rendering the valve means operableaccording to the width of the printed circuit board, and an adjustableflow control means in fluid communication with said ejection nozzles andhaving an inlet in fluid communication with the discharge of the pumpand an outlet in fluid communication with the intake of the pump, saidflow control means adjustable for controlling the rate of the flow ofthe molten wax through the ejection nozzles by diverting a controlledamount of the molten wax through the inlet and out of the outlet.
 2. Anapparatus as claimed in claim 1, further comprising apressure-responsive safety valve having an intake port in fluidcommunication with the discharge of the pump and an outlet port in fluidcommunication with the intake of the pump and operable for diverting anamount of the molten wax discharged from the pump to the intake of thepump through the intake port and the outlet port when the pressure ofthe molten wax flowing through the discharge side of the pump exceeds apredetermined pressure.
 3. An apparatus as claimed in claim 2, furthercomprising means for supporting the circuit board for travel along thetransfer path, the support means being adjustable to accommodate circuitboards of different widths, and the selective valve operating meansincluding actuation means coupled to the support means to controloperation of selected ones of the valve means in response to thewidth-adjusted position of the support means.
 4. An apparatus forapplying molten wax onto a printed circuit board travelling along apredetermined transfer path, comprising:a vessel for containing a moltenwax; a manifold integrally attached to the vessel and having formedtherein a U-shaped fluid flow passage, a bore extending between firstand second end portions of the U-shaped passage and a plurality ofdischarge ports each being in fluid communication with the U-shapedpassage; a gear pump mounted on the manifold and having a suction sideopening into the vessel and a discharge side opening into the first endportion of the the U-shaped passage; a return passage defined within themanifold and extending betwen the suction side of the gear pump and anintermediate portion of the bore; an integrated pressure-responsivesafety valve and flow control valve assembly disposed within the bore,said safety valve being operable when the pressure within the U-shapedpassage exceeds a predetermined pressure for allowing a part of themolten wax to flow through the bore, from the first end portion of theflow passage to the return passage, said control valve being operablefor controlling the rate of the flow of the molten wax through theU-shaped passage by diverting a controlled amount of the molten wax tothe return passage; and a plurality of ejection means connected torespective discharge ports of the manifold for ejecting the molten waxtherefrom towards the printed circuit board.
 5. An apparatus as claimedin claim 4, further comprising means for controlling operation ofselected ones of the plurality of ejection means according to the widthof the printed circuit board.
 6. An apparatus as claimed in claim 5,further comprising means for supporting the circuit board for travelalong the transfer path, the support means being adjustable toaccommodate circuit boards of different widths.
 7. An apparatus asclaimed in claim 6, wherein the selective valve operating control meansincludes activation means coupled to the support means to controloperation of selected ones of the plurality of ejection means accordingto the width-adjusted position of the support means.